Vitamin B12 (cobalamin) deficiency is a significant public health problem among the elderly. Deficiency can lead to a wide spectrum of hematological and neurological disorders (dementia) which are usually reversible by early diagnosis and prompt treatment. The primary cause of deficiency is malabsorption due to a lack of intrinsic factor (pernicious anemia, PA) or gastric atrophy. Accurate diagnostic tests for PA and other causes of B12 malabsorption are needed because symptoms attributable to B12 deficiency, neurological dysfunction and dementia, are non-specific and difficult to diagnose. The currently used procedure for assessing B12 malabsorption is the Schilling Urinary Excretion Test, first introduced in 1953. While this radioisotopic test led to great advances in understanding B12 related disease, it is imprecise and cumbersome and its manufacture was terminated in 2003 because of safety concerns regarding the test components. As a result, the underlying causes of B12 deficiency are frequently not addressed, despite important health implications and potential economic costs of establishing a cause of B12 deficiency. This proposal describes a new B12 absorption test made possible by two key technological innovations: an inexpensive biosynthetic system for production of isotopic (14C) B12 (Tracer B12), and ultra sensitive detection of the Tracer B12 by Accelerator Mass Spectrometry, a tool that achieves zeptomole (10-21) sensitivity for detection of 14C, the greatest sensitivity of any known analytical instrument. Using this approach, the absorption of an oral dose of Tracer B12 can be quantified in a single drop of blood using harmlessly small radioactive doses (25-50 nanoCuries). Our overarching objective is to develop a test that would replace the Schilling test to detect malabsorption of B12 and correctly identify the cause of the malabsorption. For Phase II of this project, our specific aims are: 1) To define and quantify the normal and abnormal ranges of plasma response to an oral dose of Tracer B12. To achieve this aim, we will carry out a kinetic study of Tracer B12 in 10 age- matched pairs of healthy controls and PA patients. 2) To confirm the cause of B12 malabsorption among PA patients by assessing the changes in plasma response to an oral dose of Tracer B12 combined with recombinant human intrinsic factor (rh-IF) in PA patients. To achieve this aim, the 10 PA patients from specific aim 1 will receive a second oral dose of Tracer B12 combined with rh-IF, and the time course and variability of Tracer B12 appearance and disappearance in plasma and whole blood will be re-assessed. We predict that rh-IF will correct the malabsorption in PA patients. This Phase II application follows upon the successful completion of Phase I in which we synthesized sufficient Tracer B12 to support the clinical component of the proposal. The work has significance as it should lead to a convenient, quantitative and most needed diagnostic test for clinical B12 malabsorption, and appropriate treatment for B12 deficiency disorders. PUBLIC HEALTH RELEVANCE: This proposal describes a new test method made possible by two key technological innovations: an inexpensive biosynthetic system for production of isotopic (14C) Vitamin B12 (Tracer B12), and an ultra sensitive detection of the Tracer B12 by Accelerator Mass Spectrometry. The work is significant as it would lead to a needed convenient diagnostic test for B12 malabsorption that can be employed in the clinic or in research settings.